The present invention relates to a humeral prosthesis. More particularly, the present invention relates to a series of humeral stems, each stem head having an axial bore and longitudinal slots along the distal end to allow for distal compression. The compression force necessary to compress the distal diameter of the stem varies by stem diameter. Thus, as the stem diameter increases, the compression force decreases, thereby relieving cortical wall pressure in potentially thinner cortical walls.
Shoulder replacement operations involve replacing at least a portion of the proximal section of the humeral shaft with a prosthesis. Typically, the medullary canal of the humerus is reamed or bored at its upper end for receiving the prosthesis. A stem portion of the prosthesis is inserted into the reamed portion of the humerus in a secure position. The stem engages an artificial humeral head for receipt by the glenoid cavity.
Early shoulder prostheses were typically unitary structures including a stem, to be implanted within the humerus, and a head, to be positioned within the glenoid cavity of the scapula, attempting to directly mimic the unitary structure of the upper portion of the humerus that they were designed to replace.
Due to several problems with the a conventional unitary shoulder prosthesis, among them the necessity of maintaining a large inventory of differently configured prostheses in order to accommodate the varying morphologies of patients, assorted modular prostheses have been developed. A modular prosthesis generally consists of two parts: a stem that is mounted into the medullary canal of the humerus, and a head component connected in some manner to the stem. The head component replaces the bearing surface of the scapula to allow the movement of the shoulder. Different stem sizes and head sizes in a modular prosthesis provide the surgeon with some degree of inter-operative flexibility which facilitates reconstruction of the original anatomy of the patient. With a range of stem sizes and a range of head sizes available, the surgeon can choose a particular combination to suit the anatomy of each individual without having to have a large inventory of unitary humeral prostheses.
While modular prostheses have solved a number of problems prevalent in the prior art, others remain. For example, unitary as well as modular prostheses typically fail to maximize flexibility of the stem portion in order to reduce hoop stress during insertion of the stem.
The present invention relates generally to shoulder prostheses. In particular, the invention provides a series of humeral stems having increased stability and distal flexibility to relieve humeral pain. Generally, the stems are for use in a modular shoulder prosthesis. However, if desired, a unitary prosthesis could be designed having a stem configured as taught herein. The stem has utility in any prosthesis in which a stem of the prosthesis is to be inserted into the intermedullary canal of the bone, such as a shoulder, hip, knee, or finger prosthesis.
The modular humeral prosthesis generally comprises a stem to be fitted to a resected humerus and a head sized and configured to approximate the humeral head. Optionally, one or more connecting members may be used to connect the stem to the head in a variety of configurations. For example, the connecting member could be used to achieve varying degrees of eccentricity (offset) or tilt of the head to the stem. The humeral head has a spherical surface on one side and a flat face on the opposite side. The spherical surface replaces the bearing surface of the normal humeral head to allow movement of the shoulder
Each stem of the series is shaped for insertion into the reamed portion of the medullary canal of the humerus. Each stem includes an axial bore and a series of longitudinal slots are spaced around the distal portion of the stem allow for distal compression. The size and length of the bore as well as the length of the slots are codependent and determine the compression force necessary to compress the distal diameter of the stems by a predetermined amount. The bore and slots are sized and configured such that the compression force needed to compress each stem decreases as the diameter of the stem increases. Preferably, a series of stems is provided having varying diameters.
Thus, the stem reduces hoop stress during insertion of the stem. The stem is especially effective in relieving cortical wall pressures in potentially thinner cortical walls.